In weaving looms, and particularly in jacquard looms, heddles are moved in one direction while being pulled by a spring in the other direction. Generally, the heddle is moved by the spring to form the lower shed. The spring is anchored at its opposite end in stationary fashion in the loom or to the floor and keeps a harness cord and the heddle under tension during their operation.
Like any spring-elastic system, the assembly comprising the spring, heddle and harness cord also exhibits a resonance phenomena, including the propagation of undulations that pass through the linear system. The natural resonance of the system does not matter, as long as the rate of motion of the heddle is low compared to the resonant frequency. However, at the moment when the rate of motion of the heddle reaches the range of the resonant frequency, unwanted undulations occur in the spring. The undulations are induced in the spring by the motion of the heddle, and they travel toward the fixed end, where they are reflected and run back toward the heddle.
Under unfavorable circumstances, the heddle can even lose tension, since the returning undulation in the connection between the spring and the heddle has a phase relationship counter to the motion initialized by the motion of the harness cord.
The resonance inside the spring also causes increased mechanical stress and premature breakage.
To damp the resonance in the spring, it is known from European Patent Disclosure EP 0 678 603 to provide the lower spring fastening point with a damping device. The lower spring fastening point comprises a plastic molded part with a threaded peg onto which the helical spring is screwed. The threaded peg has two legs on its free end that are spring-elastically moveable counter to one another and which protrude into the interior of the spring and press against the spring. On the end remote from the threaded peg, the two legs are joined together again and merge with two further legs, which form an open fork.
It has been found that this type of spring damping is not unproblematic. If the contact pressure with which the legs act against the inside of the spring windings is too hard, no effective damping action occurs. Instead, the arriving undulations are reflected, largely unattenuated, at those points where the legs touch the inside of the spring. Conversely, if the contact pressure is too low, once again adequate damping does not ensue. This unfavorable phenomenon is reinforced by the fact that the spring elasticity of the plastic can exhibit fatigue and is also temperature-dependent. Furthermore, it is difficult to thread the open ends of the legs into the spring.